1. Field of the Invention
This invention relates to the field of focusing electron beams in cathode ray tubes, and in particular, to a permanent magnet focus unit for a cathode ray tube having an astigmatism corrector assembled integrally therewith.
2. Description of Related Art
A typical cathode ray tube 2 shown in FIG. 1 has an electron gun 4 for generating an electron beam 6 which strikes a screen 8 of the cathode ray tube. The electron beam is deflected for generating a raster by a deflection yoke 10. The electron beam is focused by a focus coil or assembly 12 which generates a magnetic focus field 16, shown in FIG. 2.
The magnetic field 16 can be produced by an annular wound coil, a toroidal permanent magnet, an array of permanent magnets, or a combination thereof, which alone or together generate a coaxial magnetic field. The magnetic field acts as a lens for the electron beam. This lens tends to have the same aberrations as would a lens for light rays, for example astigmatism.
It is possible to correct astigmatism of the electron beam by the way of additional coils which are generally denoted astigmatism correctors. The necessary magnetic field is generally produced by four windings coupled to have alternately North and South magnetic poles. It is possible to correct horizontal or vertical beam astigmatism by changing the current direction in the winding. Diagonal coils can be used in a similar fashion to correct the diagonal astigmatism. Axial and diagonal astigmatism correction are illustrated in FIGS. 3 and 4 respectfully The currents in the windings exert forces (F). The correctors are designed as quadripolar lenses for effecting axial and diagonal astigmatic corrections, as shown in FIGS. 5 and 6 respectively.
Generally, the astigmatism corrector is designed as a winding in a separate unit or assembly positioned at the outlet of the electron gun, as shown in FIG. 1. This arrangement entails several disadvantages. Firstly, the sensitivity is generally low because of the small beam diameter at the outlet of the electron gun. Secondly, this technology is generally used to correct the electron gun astigmatism. When such a corrector is combined with a magnetic focus coil, the astigmatism correction is not easy to obtain because of the beam rotation imparted by the focus coil. Thirdly, the cost of such a solution can be significant due to the need for designing and manufacturing a separate unit or assembly.